On 16.05.2011 03:13, Hyrum K Wright wrote:
> Several places in wc_db we use the following pattern to select all
> nodes with a common tree ancestor:
> WHERE wc_id = ?1 AND (local_relpath = ?2 OR local_relpath LIKE ?3 ESCAPE '#')
> While this works, there was some concern about whether or not SQLite
> was using the proper indicies when executing this query. By examining
> the output for 'EXPLAIN QUERY PLAN' on some of the relevant SELECT
> statements, I believe it does use the indicies as intended.
> However, I stumbled across an alternate implementation which I believe
> has some merit. Instead of the above clause, we could use:
> WHERE wc_id = ?1 AND substr(local_relpath, 1, length(?2)) = ?2
> This also avoids a table scan by making use of the indicies, but has
> the advantage of not having to compute a separate parameter for the
> LIKE clause in C. It returns the same results, and has the benefit of
> being a bit more clear to SQLite what we're trying to accomplish. I'm
> tempted to switch our code to using this new format, but wanted some
> comments first. I have not yet run extensive timing or other analysis
> on the performance.
Can't be right. I'm assuming the first query works correctly iff:
?2 = foo
?3 = foo/%
and returns 'foo' and all its subtree.
The second query can't return the same results; if ?2=foo, it'll match
foobar, which is not foo's child; if ?2=foo/, it won't return foo.
Received on 2011-05-16 04:09:51 CEST